The present invention is directed to aqueous alkali metal cleaning compositions. In particular, the present invention is directed to aqueous alkali metal cleaning compositions containing water soluble salt builders and alkali metal silicate anticorrosion agents which are readily water soluble at pH levels below 11.0. The compositions of the present invention are useful in so-called parts washers which are particularly adapted for industrial cleaning, as well as for domestic use.
Parts washers of various kinds are known to those skilled in the art as having great utility for mechanics and others working in a variety of occupations, particularly those working in industrial plants, maintenance and repair services, and the like. Parts washers referred to herein include soak tanks, so-called hot tanks, immersion type parts cleaners with or without air agitation, spray washers (continuous or batch) and ultrasonic baths. Generally, parts washers are used to remove all types of contaminants adhered to metal surfaces including greases, cutting fluids, drawing fluids, machine oils, antirust oils such as cosmoline, carbonaceous oils, sebaceous oils, particulate matter, waxes, paraffins, used motor oil, fuels, etc.
Until recently, metal surfaces were cleaned of most oily and greasy contamination by use of solvents. Existing solvents, with or without special additives, are adequate to achieve good cleaning of most dirty, greasy, metal parts. A great number of solvents have been employed to produce metallic surfaces free from contamination. These wash solvents generally include various halogenated hydrocarbons and non-halogenated hydrocarbons, of significant quantity industry wide for cleaning and degreasing of metal surfaces, and the degree of success with each of these wash solvents generally is dependent upon the degree of cleanliness required of the resultant surface.
Recently, however, various hydrocarbon and halogenated hydrocarbon metal cleaning solvents previously employed have come under scrutiny in view of the materials employed, and in particular, the environmental impact from the usage of the various materials. This is particularly true in the case of parts cleaning which is done in closed environments such as garages and the like or for even home usage in view of the close human contact. Even addition of devices to parts washers which can reduce spillage, fire and excessive volatilization of cleaning solvents are not sufficient to alleviate present environmental concerns.
Although halogenated hydrocarbon solvents such as chlorofluorocarbons (CFCs) and trichloromethane, methylene chloride and trichloromethane (methyl chloroform) are used widely in industry for metal cleaning, their safety, environmental and cost factors coupled with waste disposal problems are negative aspects in their usage. A world-wide and U.S. ban on most halogenated hydrocarbon solvents is soon in the offing by virtue of the Montreal Protocol, Clean Air Act and Executive and Departmental directives.
Non-halogenated hydrocarbon solvents such as toluene and Stoddard solvent and like organic compounds such as ketones and alcohols on the other hand are flammable, have high volatility and dubious ability to be recycled for continuous use. Such properties plus unfavorable safety, environmental and cost factors, put this group of solvents in a category which is unattractive for practical consideration. Most useful organic solvents are classified as volatile organic compounds (VOCs) which pollute the atmosphere, promote formation of toxic ozone at ground level, and add to the inventory of greenhouse gases.
In order to eliminate the various negative aspects of known chemical washing and degreasing systems, it has, therefore, been suggested that an aqueous detergent system be used to overcome some of the inherent negative environmental and health aspects of such solvent cleaning systems. Unfortunately, aqueous cleaning systems are not without their own problems as related to use thereof in metal cleaning systems including use in parts washers as described above. For example, certain aqueous cleaners are exceedingly alkaline having pHs of 13 and above such as sodium hydroxide or include organic solvents such as alkanolamine, ethers, alcohols, glycols, ketones and the like. Besides being highly corrosive, exceedingly high alkaline aqueous solutions are highly toxic and can be dangerous to handle requiring extreme safety measures to avoid contact with skin. Organic solvent-containing aqueous cleaners present problems regarding toxicity, volatility or the environment as expressed previously. On the other hand, it is most difficult to obtain an aqueous detersive solution at moderate pH which is effective in removing greases and oils which contaminate metal and which would not be corrosive to metal.
One particular disadvantage of using aqueous systems to clean metal surfaces is a potential for corroding or discoloring metal surfaces. While aqueous cleaning solutions having a high pH such as formed from sodium hydroxide are often more corrosive than aqueous solutions having a relatively low pH, such as formed by moderately alkaline detergents, corrosion and discoloration still are problematic with more moderate solutions. Various corrosion inhibitors are known and have been used to prevent corrosion of surfaces which come into contact with aqueous alkaline solutions. Probably, the most effective and least costly of the known corrosion inhibitors are the silicates, such as alkali metal silicates. Unfortunately, alkali metal silicates begin to precipitate from aqueous solution at pHs below 11. This is true especially in aqueous concentrates. Such insolubility of alkali metal silicates greatly reduces effectiveness of such silicates to prevent corrosion of metal surfaces when used in aqueous cleaning solutions having a moderate pH. To prevent precipitation of silicate from aqueous concentrates below a pH of 11.0, acrylic acid polymers having a high molecular weight, i.e., 100,000 or greater, have been added to concentrates. A disadvantage to employing such polymers is their high viscosity which often leaves a hard residue along the sides of cleaning tanks. Removal of such residue can be both time consuming and difficult.
Another problem associated with aqueous cleaning compositions is solubility of builders in water. Alkali metal carbonates are especially desirable builders because such compounds are effective in removing grease and other contaminants from metals and are environmentally benign. However, alkali metal carbonates can be very difficult to solubilize in aqueous compositions, and to maintain soluble in aqueous compositions for periods of prolonged storage, thus compromising use of such builders in the cleaning industry.
Accordingly, to be as effective and be able to replace halogenated and hydrocarbon solvents now widely used, aqueous metal cleaning compositions will have to be formulated to solve the problems associated therewith including solubility of builders and anticorrosion agents, efficacy of detersive action at moderate pH levels, and corrosiveness inherent in aqueous based systems, in particular, on metal substrates.
A primary objective of the present invention is to provide an aqueous metal cleaning composition where silicate salts are readily water soluble and remain stable in solution at moderate pH ranges.
Another objective of the present invention is to provide an aqueous metal cleaning composition where carbonate salts are readily water soluble and remain in aqueous solution over prolonged storage periods.
Still another objective of the present invention is to provide an aqueous metal cleaning composition which can be used effectively in a variety of parts washing equipment to efficiently remove grease, oil and other contaminants from metal parts and which is safe to use and not a hazard to the environment in use or upon disposal, and is not irritating to human skin.
A further objective of the present invention is to provide an aqueous metal cleaning composition which contains an effective water soluble corrosion inhibitor.